1. Field of the Invention
This invention relates to a manufacturing method for precision glass molded products having a fine structure such as an optical fiber fixing member with a high size accuracy by correcting errors in size occurring due to thermal shrinkage differences between the glass molded products and the mold caused during a cooling process after pressurized molding.
2. Description of Related Art
An optical fiber used for optical communication is generally a glass made fine fiber. For example, a quartz single mode optical fiber used for long distance optical communication is constituted of a core having an outer diameter of about 10 micron meters and a clad covering the core and having an outer diameter of 125 micron meters. A quartz multi-mode optical fiber is constituted of a core having an outer diameter of 50 to 100 micron meters and a clad covering the core and having an outer diameter of 125 micron meters. Accordingly, high alignment precision is required to reduce a connection loss at optical connecting points when optical fibers are optically connected to each other or when an optical fiber is optically connected to an optical device such as an optical waveguide, a lens, an light emitting device, a photo-receiving device, etc. In particular, optical connections between quartz single mode optical fibers and between a quartz single mode optical fiber and a quartz glass single mode optical waveguide require a high alignment precision of around .+-.1 micron meter.
To optically connect an optical fiber with another optical fiber or optical device, the optical fiber is fixed in advance by an optical fiber fixing member such as optical connector or optical fiber array. The optical fiber array here means a member at least including an optical fiber guide block and a fiber fixing lid. The optical fiber guide block is made of a thin plate formed with engagement portions for fixing optical fibers to position the optical fibers. The fiber fixing lid is made of a thin plate for pressing the optical fibers engaging with the engagement member to fix the optical fibers. For example, Japanese Unexamined Patent Publication, Heisei No. 7-5,341 discloses an optical fiber array for fixing a tape fiber in which plural optical fibers arranged in a row are protected by a resin cover.
FIG. 1 shows an optical fiber array disclosed in FIG. 2 in above Japanese Unexamined Patent Publication, Heisei No. 7-5,341. This optical fiber array 200 includes an optical fiber guide block 204 in a thin plate shape on which a prescribed number of V-shaped grooves 203 serving as engagement portions for fixing optical fibers are formed for fixing optical fibers 202 striped from a tape fiber 201, and a fiber fixing lid 205 for optical fibers in a thin plate shape to press the optical fibers to immobilize the fibers engaged with the V-shaped grooves 203. The optical fiber guide block 204 constituting the optical fiber array 200 has, in addition to the V-shaped grooves 203, a seat 207 for securing a covered portion 206 of the tape fiber 201. The seat 207 is formed at a position lower than the V-shaped grooves 203. The optical fiber array 200 includes a holding block 208 having a prescribed cross section to securely hold the covered portion 206 fixed at the seat 207.
Glass, ceramic, silicon, resin, etc. are used as a material constituting a member for fixing optical fibers (hereinafter referred to as "optical fiber holder") such as optical fiber array or the like. Ultraviolet ray setting type adhesives having good property for work are desirable for fixing the fiber fixing lid on the optical fiber guide block and for connecting the optical fiber array with other optical fiber array or the like. Therefore, glasses having good ultraviolet permeability are getting favored as a material for optical fiber arrays. An optical fiber guide block required to have a high precision in size at optical fiber engagement portions, among glass made optical fiber holders, has been fabricated by mechanically processing a glass block and the like in use of a dicing saw, diamond hone, etc. Such a fabrication process, however, raises a problem about mass production, production costs, and yields.
A method applying a method for molding optical glass lens has been proposed as a mass production method for optical fiber holder with lower costs. For example, Japanese Unexamined Patent Publication, Heisei No. 6-201,936 discloses a method for pressing a transparent material such a glass plate or the like with a high temperature by a mold having projections for forming grooves. However, no detail of the method is disclosed.
Japanese Unexamined Patent Publication Heisei No. 8-211,244 discloses a molding method for optical fiber holder using a glass containing no lead and having a low softening point. This publication, however, lacks a detailed description about means for improving precision in size of optical fiber engagement portions and has no description about precision in size of the optical fiber engagement portions of the obtained optical fiber holder.
On the other hand, Japanese Unexamined Patent Publication Heisei No. 7-218,739 discloses that a pitch precision of molded optical fiber engagement portions is highly precise, less than .+-.0.5 micron meter. The optical fiber engagement portions of the optical fiber holder are formed, as shown in FIG. 3, to have a distance of fiber's center to center (l.sub.1 to l.sub.7) of, e.g., 250 micron meters; a permissive range of vertical and horizontal deviations of fiber's center is .+-.0.5 micron meter; each fiber must be positioned within this permissive range. As the number of the optical fiber engagement portions increases, the permissive range of pitch precision at the projection becomes smaller, so that a higher precision in size is required. The Publication above, however, includes no description except that the pitch precision at projections is 0.5 micron meter or less to form the obtained optical fiber engagement portion.
It is not so rare for molded glass in fact to have, when glass is molded with high temperature, difference in size between the mold and the molded product of 3 micron meters or above according to molding conditions such as a combination of glasses and molds, pressures, etc. Accordingly, within the range disclosed by Japanese Unexamined Patent Publication Heisei No. 7-218,739, it would be impossible in a practical sense to mold with a high temperature a glass product having permissible range of .+-.0.5 micron meter with respect to pitch shifts or depth shifts.